This invention relates to a developing apparatus for an electrostatic copying machine, and more particularly to a dry or powder type developing apparatus employing a single component developer.
In general, in electrostatic copying machines, a photosensitive member is charged. The photosensitive member then is exposed to a light image corresponding to the original subject matter to form a latent electrostatic image on the photosensitive member. In accordance with the electric potential of the latent electrostatic image, the developing apparatus causes an electrically charged developer to adhere to the latent electrostatic image on the photosensitive member. The developed image is then transferred to and fixed on paper. In this manner, copies are obtained by a typical electrostatic copying machine.
As to the developing apparatus in such electrostatic copying machines, there are two types. The first type uses a double component developer comprising a toner and a carrier, and the other type uses a single component developer comprising only a toner, that is, a developer without a carrier. In the first type of developing apparatus, it is necessary to incorporate a toner concentration controlling device for keeping the mix ratio of the toner and the carrier constant. Thus, this type of developing apparatus has several shortcomings, such as a complicated mechanism, large size and expensive to manufacture.
The present invention relates in particular to the other type of developing apparatus employing a single component developer. Single component developers include either toner powders comprising resins and pigments or magnetic toners comprising a mixture of resins and magnetic iron powders or resins containing magnetic powders.
A developing apparatus using a single component developer is shown in U.S. Pat. No. 4,100,884 (Mochizuki et al). The developing apparatus disclosed in FIG. 2 of this patent comprises a movable rubber roller, a plurality of magnets disposed within the hollow space of the roller, a hopper disposed above the roller which contains a quantity of magnetic toner, a levelling member disposed next to the hopper close to or in gentle contact with the roller surface and a triboelectric charger positioned in pressure contact with the movable rubber roller. With the rotation of the rubber roller, magnetic toner particles are carried out of the hopper to form a toner layer on the roller surface. The toner layer is levelled by the levelling member and then scrubbed by the triboelectric charger to charge the toner layer to a predetermined polarity. The charged particles of the toner layer are then conveyed into contact with a latent electrostatic image on an image bearing or photosensitive member.
The above apparatus has several shortcomings. The toner particles often are dropped or scattered at the triboelectric charger because a gap is formed between the rubber roller and the triboelectric charger. The gap occurs because the triboelectric charger, which is brought into contact with the rubber roller for sufficient triboelectric charging of the toner, extends beyond the contact point with the rubber roller. As a result, many stray toner particles adhere to the latent electrostatic image on the image bearing member. This shortcoming can be overcome by increasing the contact pressure of the triboelectric charger against the rubber roller; however, the toner particles coagulate or adhere to the surface of the rubber roller due to the increased contact pressure of the triboelectric charger. As a result, it is impossible to selectively separate the toner particles from the surface of the rubber roller by the electric attraction of the latent electrostatic image formed on the image bearing or photosensitive member. Accordingly, good images cannot be obtained by this developing apparatus.
Finally, in order to uniformly charge the toner layer, the triboelectric charger must make uniform contact along the longitudinal axis of the rubber roller. It is very difficult with the above described apparatus to achieve uniform contact between the triboelectric charger and the rubber roller.